The present invention relates generally to the field of thermo-optic systems, such as systems used for thermal imaging.
In the field of thermal imaging systems, it is known to employ certain device types and architectures to realize a desired function. For example, it is known to employ arrays of thermally sensitive devices along with other circuitry in thermal imaging systems, such as commonly used in military applications. Because of the low signal-to-noise ratios commonly found in operating environments, some systems employ cooled arrays of semiconductor sensor devices to reduce the noise associated with ambient thermal conditions, improving optical performance. However, such systems have typically been very expensive and complex, and thus their utilization has been limited to military and other applications that are relatively cost-insensitive.
Uncooled thermal imaging systems are also known. Such uncooled systems generally are constructed using micro-electrical-mechanical systems (MEMS) technology to create an array of thermally isolated microstructures electrically and mechanically interconnected to a custom CMOS readout. The CMOS readout generates a signal from each microstructure element and creates an electronic or visible image. Such uncooled systems generates high quality imagery but remain expensive due to the manufacturing complexity of fabricating a microstructure device integrated with a custom CMOS readout device. The use of electrical interconnects also degrades the performance of the device due to the thermal conductivity of the interconnects.
The use of optical readouts rather than CMOS readouts are advantageous as they do not require either an interconnected CMOS device or the use of electrical interconnects. However, optical readouts typically have low contrast due to large optical background signals generated when optically probing the thermal microstructure device. Signals from optically readout devices may also contain substantial non-uniformities that result from variations in manufacturing processes and readout techniques. The net result of all such background signals is reduced dynamic range and lower sensitivity.
A paper by J. Zhao entitled “High Sensitivity Photomechanical MW-LWIR Imaging using an Uncooled MEMS Microcantilever Array and Optical Readout” Proceedings of the SPIE, Vol. 5783, pp. 506-513, 2005 shows the use of a bi-material cantilever approach (deforms with temperature), along with a mechanically based way of compensating for manufacturing differences in films and ambient temperature. It appears that there are two segments to the bi-material cantilever section, one of which is thermally isolated and the other which is not, and these cause opposite bending on the reflector area. This will compensate to the degree the two segments can be made to behave identically mechanically.
Y. Zhao, M. Mao, R. Horowitz, A. Majumdar, J. Varesi, P. Norton and J. Kitching, “Optomechanical Uncooled Infrared Imaging System: Design, Microfabrication, and Performance” Journal of Microelectromechanical Systems, Vol. 11, No. 2, Apr. 2002 shows an approach based on bi-material cantilevers, some of which are metallized and some not. Even at ambient conditions, the different elements are already at significantly different levels, and therefore cause a high baseline optical level. This effect is extremely difficult to control because it is dependent on minute stresses in the films composing the structure, and moreover there will be a very large effect from ambient temperature. The authors conclude that their inability to control the substrate temperature to within better than 20 mK was the limitation on their system performance.
U.S. Pat. No. 6,124,593 to Bly et al. shows the use of thermo-mechanical Fabry-Perot filters as a readout for incident thermal radiation.
U.S. Pat. No. 6,766,882 to Carr et al. shows the use of pyro-optical material based pixels which change their absorption of visible or NIR carrier signal according to temperature, which is modulated by thermal radiation.
U.S. Pat. No. 4,594,507 to Elliott et al. shows an optically-read thermal imaging system. In this case, a “chopper” is used on the thermal radiation in order to modulate it temporally, and then an electronic readout system is used that can “lock in” to the modulation frequency and therefore extract the signal alone from signal +baseline.
US 2007/0023661 to Wagner et al. shows the use of thermo-optic materials in thin film interference filters which change their transmissive and reflective properties according to temperature, and the use of these devices to measure temperature in application including thermal imaging.